Toll-like receptor 9 in systemic sclerosis patients: relation to modified Rodnan skin score, disease severity, and functional status

Anti-Th/To Antibodies in Scleroderma: Good Prognosis or Serious Concern?

Systemic sclerosis (SSc) represents a rare and intricate autoimmune connective tissue disease, the pathophysiology of which has not been fully understood. Its key features include progressive fibrosis of the skin and internal organs, vasculopathy and aberrant immune activation. While various anti-nuclear antibodies can serve as biomarkers for the classification and prognosis of SSc, their direct role in organ dysfunction remains unclear. Anti-Th/To antibodies are present in approximately 5% of SSc patients, and are particularly prevalent among those with the limited subtype of the disease. Although the presence of these autoantibodies is associated with a mild course of the disease, there is a strong connection between them and severe clinical manifestations of SSc, including interstitial lung disease, pulmonary arterial hypertension and gastrointestinal involvement. Also, the additional clinical correlations, particularly with malignancies, need further research. Moreover, the disease's course seems to be influenced by antibodies, specific serum cytokines and TLR signaling pathways. Understanding the relationships between presence of anti-Th/To, its molecular aspects and response to treatment options is crucial for the development of novel, personalized therapeutic techniques and should undergo profound analysis in future studies.

Mitochondrial DNA Sensing Pathogen Recognition Receptors in Systemic Sclerosis Associated Interstitial Lung Disease: A Review

Purpose of the review

Systemic sclerosis (SSc) is a condition of dermal and visceral scar formation characterized by immune dysregulation and inflammatory fibrosis. Approximately 90% of SSc patients develop interstitial lung disease (ILD), and it is the leading cause of morbidity and mortality. Further understanding of immune-mediated fibroproliferative mechanisms has the potential to catalyze novel treatment approaches in this difficult to treat disease.

Recent findings

Recent advances have demonstrated the critical role of aberrant innate immune activation mediated by mitochondrial DNA (mtDNA) through interactions with toll-like receptor 9 (TLR9) and cytosolic cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS).

Summary

In this review, we will discuss how the nature of the mtDNA, whether oxidized or mutated, and its mechanism of release, either intracellularly or extracellularly, can amplify fibrogenesis by activating TLR9 and cGAS, and the novel insights gained by interrogating these signaling pathways. Because the scope of this review is intended to generate hypotheses for future research, we conclude our discussion with several important unanswered questions.

Fibronectin-EDA accumulates via reduced ubiquitination downstream of Toll-like receptor 9 activation in SSc-ILD fibroblasts

Accumulation of excessive extracellular matrix (ECM) components from lung fibroblasts is a feature of systemic sclerosis-associated interstitial lung disease (SSc-ILD), and there is increasing evidence that innate immune signaling pathways contribute to these processes. Toll-like receptors (TLRs) are innate immune sensors activated by danger signals derived from pathogens or host molecular patterns. Several damage-associated molecular pattern (DAMP) molecules are elevated in SSc-ILD plasma, including ligands that activate TLR9, an innate immune sensor recently implicated in driving profibrotic responses in fibroblasts. Fibronectin and the isoform fibronectin-extra domain A (FN-EDA) are prominent in pathological extracellular matrix accumulation, but mechanisms promoting FN-EDA accumulation are only partially understood. Here, we show that TLR9 activation increases FN-EDA accumulation in MRC5 and SSc-ILD fibroblasts, but that this effect is independent of changes in FN-EDA gene transcription. Rather, we describe a novel mechanism where TLR9 activation inhibits FN-EDA turnover via reduced FN-EDA ubiquitination. TLR9 ligand ODN2006 reduces ubiquitinated FN-EDA destined for lysosomal degradation, an effect abrogated with TLR9 knockdown or inhibition. Taken together, these results provide rationale for disrupting the TLR9 signaling axis or FN-EDA degradation pathways to reduce FN-EDA accumulation in SSc-ILD fibroblasts. More broadly, enhancing intracellular degradation of ECM components through TLR9 inhibition or enhanced ECM turnover could be a novel strategy to attenuate pathogenic ECM accumulation in SSc-ILD.

TLR9: A friend or a foe

The innate immune system is a primary protective line in our body. It confers its protection through different pattern recognition receptors (PRRs), especially toll like receptors (TLRs). Toll like receptor 9 (TLR9) is an intracellular TLR, expressed in different immunological and non-immunological cells. Release of cellular components, such as proteins, nucleotides, and DNA confers a beneficial inflammatory response and maintains homeostasis for removing cellular debris during normal physiological conditions. However, during pathological cellular damage and stress signals, engagement between mtDNA and TLR9 acts as an alarm for starting inflammatory and autoimmune disorders. The controversial role of TLR9 in different diseases baffled scientists if it has a protective or deleterious effect after activation during insults. Targeting the immune system, especially the TLR9 needs further investigation to provide a therapeutic strategy to control inflammation and autoimmune disorders.

Emerging targets of disease-modifying therapy for systemic sclerosis

Systemic sclerosis (SSc) has the highest cause-specific mortality of all the connective tissue diseases, and the aetiology of this complex and heterogeneous condition remains an enigma. Current disease-modifying therapies for SSc predominantly target inflammatory and vascular pathways but have variable and unpredictable clinical efficacy, and none is curative. Moreover, many of these therapies possess undesirable safety profiles and have no appreciable effect on long-term mortality. This Review describes the most promising of the existing therapeutic targets for SSc and places them in the context of our evolving understanding of the pathophysiology of this disease. As well as taking an in-depth look at the immune, inflammatory, vascular and fibrotic pathways implicated in the pathogenesis of SSc, this Review discusses emerging treatment targets and therapeutic strategies. The article concludes with an overview of important unanswered questions in SSc research that might inform the design of future studies of treatments aimed at modifying the course of this disease.

Innate immunity and Toll-like receptor signaling in the pathogenesis of scleroderma: advances and opportunities for therapy

PURPOSE OF REVIEW

Systemic sclerosis (SSc) is an autoimmune connective tissue disease in which inflammation and cytokine dysregulation leads to skin fibrosis. Toll-like receptors (TLRs) are conserved pattern recognition receptors, recognizing pathogens danger-associated molecular patterns (DAMPs) that elicit a cascade of proinflammatory signaling. Recently, TLRs have been found to be critically important in SSc pathogenesis, with increased levels of the TLRs and their ligands present in the disease. Animal models have also been pivotal in delineating the role of these innate immune receptors in SSc. This current review examines the role of TLRs and the most recent evidence of the role of DAMPs and how these may be exploited therapeutically.

RECENT FINDINGS

Increasingly, studies have demonstrated the key roles of TLR4 and other intracellular TLRs in mediating fibrosis in SSc patients and animal models. TLR4 activation appears a key point and novel DAMPs, expressed upon tissue damage, appear critical in mediating the profibrotic effect through a downstream enhancement of transforming growth factor β. Deletion of Tenascin-C or a splice variant of fibronectin ameliorates animal models of skin fibrosis. Intracellular, nucleic acid sensing, TLR8 is critical in activating macrophages to secrete profibrotic molecules. The mechanism involves histone modification through epigenetic modifying enzymes.

SUMMARY

TLRs are key therapeutic targets in SSc.

Targeting TLRs and the inflammasome in systemic sclerosis

Systemic sclerosis (SSc) is an idiopathic autoimmune disease characterised by inflammation, vascular problems, cytokine dysregulation and ultimately fibrosis, which accounts for poor prognosis and eventual mortality. At present no curative treatments exist, hence there is an urgent need to better understand the aetiology and develop improved therapies accordingly. Although still widely debated, significant evidence points to upregulation of the innate immune response via the activity of Toll-like receptors (TLRs) and the NLRP3 inflammasome as the start points in a cascade of signaling events which drives excessive extracellular matrix protein production, causing fibrosis. Herein the recent breakthroughs which have implicated TLR signaling and the NLRP3 inflammasome in SSc and the novel therapeutic possibilities this introduces to the field will be discussed.